I have a hp-67 and own the lunar lander program, however I am a novice compared to others here. I can e-mail you the code and, with a little work, it should easily port to any RPN calculator. Provided it has the indirect adressing feature.

I'd be damn astonished if a program such a lunar lander orked on a 19BII !

the loop function does not exist, as far as I know, on this calc. All you can do is to store a very powerful formula (or function) in the solver, but from there to let it loop by itself like on the 67, 41, 11C, etc... application, I really do not see how !

The Sigma function can be "abused" as a loop counter. L (let) and G (get) functions could be used to assign and read "invisible" variables.

I imagine the user could enter a value for fuel and then solve. The solver would calculate the next height, velocity etc. and stop. The user could then solve again until such time as they hit the surface. Not sure I'd want to try such a complex program contained in one line of formula, but I can't see a reason why it couldn't be done. Quite a challenge!